An Ordering Theorem for Conditionally Independent and Identically Distributed Random Variables

Abstract

Let $\mathbf{a}$ and $\mathbf{b}$ be $r$-dimensional real vectors. It is shown that if $\mathbf{a}$ majorizes $\mathbf{b}$, then $E(\Pi^r_{j = 1} X_j^a j) \geqq E(\Pi^r_{j = 1} X_j^b j)$ holds for nonnegative random variables $X_1, \cdots, X_r$ whose joint pdf is permutation symmetric. If in addition the components of $\mathbf{a, b}$ are nonnegative integers, then for every Borel-measurable set $A$, $\Pi^r_{j = 1} P\lbrack\cap^{a_j}_{i = 1} \{Z_i \in A\}\rbrack \geqq \Pi^r_{j = 1} P\lbrack\cap^{b_j}_{i = 1} \{Z_i \in A\}\rbrack$ holds for conditionally i.i.d. random variables $Z_i$. Applications are considered.